Many oil wells employ electrically driven submersible pumps to pump the well fluid to the surface. In a typical well, the pump and motor are suspended on a string of production tubing, and the pump discharges the well fluid into the tubing. The pump may be a centrifugal pump (“ESP”) having a large number of impeller and diffuser stages. A power cable extends alongside the tubing to the motor for supplying three-phase power. Progressing cavity pumps driven by downhole electrical motors are also used in some wells.
It is common for an ESP unit to have pressure and temperature sensors that transmit to the surface downhole pressure and temperature while the ESP is operating. Usually the pressure and temperature signals are superimposed on the motor power cable, and a surface electronic unit will detect the signals and provide readings.
Another useful parameter for an operator of an ESP driven well or a naturally flowing well is the flow rate of the well fluid. A variety of different flow meters exist that can be used at the surface for determining the flow rate at the surface. Downhole flow meters for wells have been employed with well surveys or production logging operations, particularly for natural pressure driven wells. In production logging, typically a downhole flow meter is lowered into the tubing on a cable. In one type, power is supplied to the flow meter through a conductor in the cable, and signals are transmitted to the surface while the well is allowed to flow. The downhole unit could be battery powered. Typically, a well survey using a flow meter is only performed periodically and for a short period of time. Normally, operators do not install downhole flow meters for continuous long term operation in ESP driven wells
The flow rate at the surface is easily measured, but may differ from a flow rate measured downhole Downhole, free gas produced by the well is more likely entrained in the well fluid or is in solution, thus will not affect a downhole flow meter reading. At the surface, much of the gas typically comes out of solution because of the lower pressure. Gassy fluid flow rates can not be accurately or easily monitored once the gas has come out of solution.